Protective devices which operate, inter alia, on the distance protection principle are used for monitoring polyphase power transmission lines for faults that occur, in particular shorts. In this case, the impedance of loops which are created by the short is determined, and this impedance is used to deduce the distance between the fault location and the measurement point. When analyzing loops such as these, a distinction must essentially be drawn between two loop types, a conductor to ground loop and a conductor to conductor loop. In the case of a conductor to ground loop, the short produces a connection between at least one of the phase conductors and ground, while in the case of a conductor to conductor loop, at least two conductors are shorted to one another without any contact with ground. Different methods are used to determine the relevant line section in the two situations. Thus, before the calculation, it is necessary to determine whether the short does or does not involve ground (short to ground).
It is known, for example, from the manual for the Siemens 7SA522 protective device, Order Number C53000-G1100-C155-2, pages 6-28 to 6-30, for earth fault identification to be carried out on the basis of sum current and residual voltage measurements for this purpose. In the known method, a fault signal which identifies a short to ground is produced when the sum current or the residual voltage is greater than a respectively predetermined threshold value (OR linking) This makes use of the effect that both a measurable sum current and a significant residual voltage normally occur only in the case of shorts to ground, but not in the case of shorts that do not involve any ground contact.
The invention relates to a method such as this and, thus, relates in particular to a method for producing a fault signal which indicates a short to ground on a polyphase power transmission line, in the case of which, once a short has occurred on the power transmission line, the phase currents in the power transmission line are detected by means of current transformers in order to form a sum current measured value, and a first short to ground suspicion signal is produced when the sum current measured value is greater than a predetermined current threshold value, the phase voltages on the power transmission line are detected in order to form a residual voltage measured value, and a second short to ground suspicion signal is produced when the residual voltage measured value is greater than a predetermined voltage threshold value, and a fault signal which indicates a short to ground is produced when at least one short to ground suspicion signal is present.
However, if at least one current transformer which is involved in the process of determining the sum current enters saturation, then the measured values are no longer detected correctly and it is possible to determine a sum current which spuriously indicates a discrepancy from zero. Using the threshold value method described above, a short to ground would then be identified, and the instruments for determining the relevant line section in the event of a short to ground would be (undesirably) enabled. This could lead to spurious tripping of a circuit breaker connected to the protective device.
In order to prevent this, so-called stabilized ground current detection and zero current/negative phase sequence system current comparison are carried out in the known device. Since the probability of the occurrence of current transformer saturation increases as the phase currents rise, the threshold value for identification of a short to ground likewise rises as the phase currents increase for stabilized ground current detection. However, in order to ensure sufficient sensitivity, the gradient of the response characteristic must be kept relatively low. In addition to the sum current, the negative phase sequence current on the power transmission line is determined for the zero current/negative phase sequence system current comparison, and a value pair is formed from these two currents. A short to ground is or is not identified depending on the locus of the value pair in a sum current/negative phase sequence current diagram.
A further known option for improving the identification of short to ground is to connect the outputs of the threshold value stages for the sum current and for the residual voltage to an AND gate (instead of to an OR gate as described above) so that a short to ground is identified only when both the sum current and the residual voltage are greater than the predetermined threshold values. However, this AND linking reduces the sensitivity of the overall method, since the residual voltage will not invariably rise above the predetermined threshold value in every situation in which a short to ground occurs.